In motion picture projectors, a succession of transparent or semi-transparent still images or frames are sequentially advanced before a suitable light source, thereby projecting an image onto a projection surface. The film is moved or advanced until a frame is in alignment with the projection aperture and then held stationary for a discrete period of time during which light is passed during two discrete time periods through the aperture, film frame, and then a focusing lens, resulting in an image being projected onto a screen. This is followed by movement of the film until the next frame is in alignment with the projection aperture. The rapid sequential advancement of the individual still frames, typically on the order of 24 frames per second, produces the illusion of motion. Typically, the film disposes at least one soundtrack synchronized with the picture projection. In this regard, recent advances in analog and digital sound recording technology have been rapidly incorporated in to modern films and film projector sound systems, more commonly referred to as “sound heads”.
To effectuate sequential film advancement, motion picture projectors utilize toothed drive sprockets connected to a motor driven “star wheel” to sequentially pull a length of film before the light source. Such motion picture projectors further include tensioning shoes for maintaining the filmstrip in contact with the drive sprockets. The film disposes two drive tracks which are engaged by the toothed sprockets, thereby enabling frame advancement. Motion picture filmstrips generally include at least one soundtrack positioned near the edge of the film or within at least one film drive track.
Commonly, motion picture projectors utilize a film trap and gate assembly to precisely align and position a filmstrip within an illumination plane. The film trap and gate assembly includes a pair of runners which provide controlled movement of the filmstrip past an illumination aperture. The edges of the filmstrip contact the runners, pressure bands, and tensioning shoes during operation. Consequently, over a period of time the edges of the filmstrip may become scratched or damaged due to the film remaining in constant contact with the runners, pressure bands, and tensioning shoes. Additionally, foreign materials such as dust, residues, and other impurities located on the runners, pressure bands, or tensioning shoes may be deposited on the film. The damage to the edges of the filmstrip or the foreign material deposited on the film may prevent the soundtracks from being clearly reproduced by the sound head.
Additionally, motion picture projectors are designed primarily for forward film advancement, in that the drive sprockets pull the filmstrip past an illumination aperture. On occasion, a film trap will include a sharpen device called a stripper plate located near the drive sprocket, to strip film splices when the film is moved through the projector in reverse. Pushing a flexible portion of film material past the sharpened stripper plate and past the runners, pressure bands, and tensioning shoes is often impossible. Moreover, severe damage to a filmstrip could occur should the filmstrip contact the stripper plate. As such, reversing the direction of the film, particularly while viewing, often results in damage to the film.
Thus, there is a need for a film trap and gate assembly for a motion picture projectors which reduces the amount of surface wear on the edges of the filmstrip, thereby increasing the lifetime of each film print. There is a further need for a film trap and gate assembly which enables the operator to reverse the direction of film travel without damaging the film.